Generally oils like vegetable, canola, peanut, etc. are not as healthy as olive oil, but I’m confused by your comment about them being worse if heated. Those oils have higher smoke points than olive oil and are preferable for certain applications like stir-frying and deep-frying (which is clearly unhealthy so may be a bit off-topic here).
Prepared/frozen meals are “less healthy” not because they specifically have oils that don’t do well with heating, but because they often have a lot of fat, sodium, and preservatives. My suggestion is to try and replace (at least some) frozen meals with simple meals made from whole ingredients, rather than trying to optimize the frozen meals themselves.
Vegetable oils are unhealthy regardless of how they’re heated. Healthy oils include olive oil, coconut oil, avocado oil, palm oil, ghee, butter, tallow, or lard. Seed oils are toxic and should be avoided as much as possible.